Pretreatment with 5-aminolevulinic acid mitigates heat stress of cucumber leaves

Cucumber seedlings were pretreated with 3 μM 5-aminolevulinic acid (ALA) followed by cultivation at normal (25/18 °C) or high (42/38 °C) day/night temperature to investigate the protective effects of ALA on heat stress in plants.Heat elevated the content of malondiadehyde (MDA), superoxide radical (O 2 ˙⎯) and hydrogen peroxide (H 2 O 2 ) in leaves of all plants but less in ALA-pretreated plants.Heat treatment resulted in higher antioxidant enzyme activities and proline and soluble sugar content and weaker growth inhibition in ALA-pretreated plants than in those treated with heat alone.These results indicate that ALA pretreatment increased the tolerance of seedlings to heat stress.